Causes of sterility in seed set of rice under salinity stress

The effects of salinity at 50 mm NaCl on floral characteristics, yield comp onents, and biochemical and physiological attributes of the sensitive rice variety IR-28 were studied under controlled conditions to determine the cau ses of sterility in seed set under salinity stress. The results showed sign ificant decreases in panicle weight, panicle length, primary branches/panic le, filled seeds/panicle, unfilled seeds/panicle, filled seeds/plant, unfil led seeds/plant, total seeds/panicle, total seed weight/panicle, 1000-seed weight and total seed weight/plant. The sodium content in different leaves and floral parts increased significantly. In contrast, the potassium conten t was decreased significantly in leaves and floral parts. A reduction in ch lorophyll a and b was also noted in different parts of the leaves. Inhibiti on of transpiration and photosynthesis was observed in flag leaves at the g rain-filling stage. Soluble carbohydrates in different leaves were reduced significantly in salinized plants but their content in different floral par ts was increased, with the exception of primary and secondary branch spikel ets. Under salinity stress, total protein concentration in flag, second and third leaves were higher than in control plants. The viability of rice pol len was reduced significantly in salinized plants. It was further observed that starch synthetase activity (alpha1-4-glucan glucosyle transferases) in developing rice grains was inhibited very significantly under salinity str ess. From these results, it is inferred that sterility and significant redu ctions in seed set in rice were not merely due to reduction or inhibition o f different biochemical constituents and physiological functions, but were mainly due to limitation of soluble carbohydrate translocation in primary a nd secondary spikelets, accumulation of more sodium and less potassium in a ll the floral parts, and highly significant inhibition of specific activity of starch synthetase in developing rice grains, thus resulting in failure of seed set.